The present invention relates to an installation of multiple flexible bottom-to-surface connections between well heads, pieces of equipment, or the ends of undersea pipes resting on the sea bottom, and a floating support on the surface, the installation comprising a multiplicity of flexible lines, in particular flexible pipes, having their bottom ends connected to the ends of a plurality of undersea pipes resting on the sea bottom or directly to well heads or to pieces of equipment resting on the sea bottom.
In the present description, the term “flexible line” is used to mean pipes or cables capable of accepting large amounts of deformation without that giving rise to significant return forces, such as the flexible pipes defined below, and also cables or pipes for transferring power or information such as electric cables, control cables, or hydraulic fluid transfer pipes powering hydraulic equipment such as actuators, or pipes containing optical fibers; a flexible line may also be a control umbilical made up of one or more hydraulic pipes and/or electric cables for transmitting power and/or information.
The technical sector of the invention is more particularly the field of fabricating and installing bottom-to-surface connections for extracting oil, gas, or other soluble or meltable material or mineral material in suspension from under the sea, via a submerged well head, and up to a floating support, in order to develop production fields located off-shore at sea. The main and immediate application of the invention lies in the field of producing oil.
In general, the floating support has anchor means enabling it to remain in position in spite of the effects of currents, wind, and swell. It also generally includes means for storing and treating oil together with off-loading means for off-loading to off-loading tankers, which generally call at regular intervals in order to remove the production. The common term for such a floating support is floating production storage off-loading, and the initials FPSO are used throughout the description below.
However it is also possible for the support to be a semi-submersible floating platform installed temporarily at sea for a few years, e.g. while waiting for an FPSO type floating support to be built and installed permanently.
Bottom-to-surface connections with an undersea pipe resting on the sea bottom are known, that are of the hybrid tower type and that comprise:                a vertical riser having its bottom end anchored to the sea bottom via a flexible hinge, that is connected to a said pipe resting on the sea bottom, and that has its top end tensioned by a float submerged in the subsurface, the top end being connected to the float; and        a connection pipe, generally a flexible connection pipe, between the top end of said riser and a support floating on the surface, said flexible connection pipe, where appropriate, taking up the shape of a catenary curve hanging under the effect of its own weight, i.e. it goes down well below the float and then subsequently rises up to said floating support.        
Bottom-to-surface connections are also known that are made by continuously raising strong rigid pipes up to the subsurface, such pipes being made of thick-walled tubular elements of steel that are welded or screwed together, and that take up a catenary configuration with curvature that varies continuously throughout the suspended length, which pipes are commonly referred to as steel catenary risers (SCR) or else as “catenary type rigid pipes” or as “SCR type risers”. Such a catenary pipe may go up as far as the support floating on the surface, or only as far as a float submerged in the subsurface that serves to tension its top end, which top end is then connected to a floating support by a hanging flexible connection pipe.
Bottom-to-surface connections are also known that enable a floating support to be connected to pipes or installations on the sea bottom that are constituted entirely by flexible pipes, in particular when the depth of water is not very great, e.g. lying in the range 300 meters (m) to 750 m, or even 1000 m, where the well heads or the pieces of undersea equipment are not very far from said floating support.
The present invention relates to flexible pipe support means for an installation of multiple bottom-to-surface connections comprising multiple flexible pipes arranged on at least two different levels in height.
It should be recalled that the term “flexible pipe” is used herein to mean pipes, sometimes also known as “hoses”, that are well known to the person in the art and that are described in standards documents published by the American Petroleum Institute (API), more particularly under the references API 17J and API RP 17 B. Such flexible pipes are manufactured and sold in particular by the supplier Technip-Coflexip France. These flexible pipes generally comprise internal sealing layers made of thermoplastic materials associated with layers suitable for withstanding internal pressure in the pipes, generally made of steel or of composite materials and used in the form of spiral-wound strips that touch one another inside the thermoplastic pipes in order to withstand internal bursting pressure, and associated with external reinforcement over the thermoplastic tubular layer and likewise in the form of touching spiral-wound strips, but using a pitch that is longer, i.e. using a smaller helix angle, particularly one lying in the range 15° to 55°.
When developing certain fields, a plurality of well heads are connected in parallel by a plurality of bottom-to-surface connections leading to a common floating support. Under such circumstances, each of said bottom-to-surface connections needs to be kept apart from its immediate neighbors in order to avoid any interference and any impacts, not only between floats, but also between flexible pipes and electric cables and other flexible lines such as electric cables or umbilicals transferring information signals and providing the connection with said floating support, when said flexible pipes are subjected to the effects of current, and when said floating support is itself subjected to swell, wind, and current.
In the development of certain fields, each of the well heads is connected individually to a said floating support and there are therefore very many bottom-to-surface connections, so it becomes impossible to install any more since the length of the side of the support is limited and as a result it can accept only a limited number of bottom-to-surface connections.
It is desired to install as many bottom-to-surface connections as possible from a given floating support in order to optimize the working of oil fields. That is why various systems have been proposed enabling a plurality of vertical risers to be associated with one another in order to reduce the occupancy of the working field and in order to be able to install a larger number of bottom-to-surface connections connected to a common floating support. Typically, it is necessary to be able to install up to 30 or even 40 bottom-to-surface connections from a common floating support.
Documents WO 02/66786, WO 02/103153, and WO 2011/061422 in the name of the Applicant describe hybrid towers with multiple flexible pipes and risers arranged in fans enabling a large number of connections to be associated with a common floating support in spite of the problem of the movements of said risers interfering with one another since they are all subjected to the same movement as their top tensioning floats under the effect of the movements of the floating support on the surface where it is subjected to swell, wind, and currents.
In those installations, proposals are made to arrange two flexible pipes that are superposed or arranged side by side between the floating support and the top ends of risers or SCRs, the two flexible pipes being guided in the subsurface by two respective troughs fastened in superposed or laterally offset manner to a float for tensioning a third riser that is located closer to the floating support than are the first two risers, each said trough thus defining two flexible pipe portions in the form of hanging double catenaries on either side of the trough. That configuration presents the advantage of making it possible to bring the flexible pipes to the top end of the riser that is relatively far from the floating support without the bottom points of said hanging double catenary pipe portions being too deep.
When a multiplicity of bottom-to-surface connections are used that are constituted exclusively by flexible pipes, it is also necessary to space the various connections apart from one another, at least for the following reasons.
Firstly, flexible pipes have fragile outer sheaths, and it is essential to prevent them from striking against one another.
Secondly, the flexible pipes are used by passing via arch-shaped guide elements referred to as “troughs”, each defining a rigid bearing surface of convex curved shape as described below, so as to define two flexible pipe portions, comprising a first flexible pipe portion in a hanging double catenary configuration between the floating support and said trough, and a second flexible pipe portion in a single catenary configuration between said trough and the point of tangential contact between said flexible pipe and the sea bottom.
Those arch-shaped guide elements referred to as troughs are well known to the person skilled in the art, they present:                a longitudinal section of curved shape in section in the axial vertical longitudinal plane of the trough, preferably a section of circular shape with its concave side facing towards the bottom of the sea, and a convex outside surface on which the pipe is placed; and        a cross-section in the vertical plane perpendicular to the vertical axial longitudinal plane of the trough presenting a shape with a curved bottom that is preferably circular with its concave side facing upwards and constituted by said top outside surface lying between longitudinal side walls serving to hold and guide the pipe in the longitudinal direction between said side walls.        
In known manner, the radius of curvature of the longitudinal curve with its concave side facing downwards is greater than the minimum radius of curvature of the pipe passing via said trough.
Such a trough serves to impart controlled curvature to the portion of flexible pipe that it supports so as to avoid excessive curvature which would irremediably damage said pipe.
The function of such troughs and the arrangement of the flexible pipes serves to create a hanging double category curve on the upstream side of the trough between the floating support and the trough so as to avoid or reduce as much as possible the stresses and movements of the flexible pipes at their point of contact with the sea floor which would destructure the sea floor by creating trenches and would weaken the pipe because of the pipe being flexed in alternation in the region of the point of contact, thereby requiring its structure to be reinforced and/or requiring the sea floor to be protected. The stresses and movements at the point of contact between the flexible pipe and the sea floor are indeed reduced as a result of the stresses and the movements of the pipe being damped by the first flexible pipe portion in the form of a hanging double catenary that is created by causing the pipe to pass over said trough, the first portion being more involved in absorbing horizontal movements of the floating support than is the second flexible pipe portion in the shape of a single catenary.
When suspended from its two ends, a said undersea flexible line takes up under its own weight the shape of a hanging double catenary, as is known to the person skilled in the art, i.e. it goes down in a catenary configuration to a low point where its tangent is horizontal (see below), after which it rises up to said floating support, which hanging catenary can accommodate large amounts of movement between its ends, which movements are absorbed by deforming the flexible pipe, in particular in the rising or descending portions on either side of the low point of said hanging catenary.
It should be recalled that the flexible pipe portion between an end from which it is suspended and the low portion of horizontal tangent, specifically in said second flexible pipe portion the point of contact with the sea bottom, adopts a symmetrical curve as formed by a hanging pipe portion of uniform weight subjected to gravity, which curve is known as a “catenary” and is a mathematical function of the hyperbolic cosine type:coshx=(ex=e−x)/2associating the abscissa and the ordinate of an arbitrary point of the curve in application of the following formulae:y=R0(cosh(x/R0)−1)R=R0·(Y/R0+1)2 where:                x represents the distance in the horizontal direction between said point of contact and a point M on the curve;        y represents the height to the point M (x and y are thus the abscissa and ordinate values of a point M on the curve relative to a rectangular frame of reference having its origin at said point of contact);        R0 represents the radius of curvature at said point of contact, i.e. the point with a horizontal tangent; and        R represents the radius of curvature at the point M(x,y).        
Thus, the curvature varies along the catenary from the top end where its radius of curvature has a maximum value Rmax to the point of contact with the sea floor where its radius of curvature has a minimum value Rmin (or R0 in the above formula). Under the effect of waves, wind, and current, the surface support moves laterally and vertically, thereby having the effect of raising or lowering the pipe of catenary shape where it touches the sea bottom.
For a bottom-to-surface connection in the form of a single catenary, the most critical portion of the catenary is situated in its portion close to the point of contact, and most of the forces in this bottom portion of the catenary are in fact generated by the movements of the floating support and by the excitations that are applied to the top portion of the catenary, which is subjected to current and to swell, with all of these excitations then propagating mechanically along the pipe to the bottom of the catenary.
The essential function of the first portion of the flexible pipe in the form of a hanging double catenary that is located upstream from the trough is thus more specifically to absorb, at least in part, the movements of the pipe and/or the movements of the floating support to which said flexible pipe is connected, by mechanically decoupling movement between respectively said floating support and said second flexible pipe portion in the form of a single catenary. However another function is also to reduce the traction forces exerted by said second flexible pipe portion on the undersea equipment and/or the end of the pipe resting on the sea bottom to which it is connected, as the case may be.
In the prior art, the intermediate support troughs for said flexible pipes are held in the subsurface at a certain depth by supporting floats from which each of the troughs is suspended. However those floats are subjected to large amounts of movement which means that sufficient distance must be provided between the various floats in order to ensure that they do not strike against one another.
In WO 00/31372 and EP 0 251 488, there are described pluralities of bottom-to-surface connections in which flexible pipes extend from a floating support to the bottom of the sea, passing via a plurality of troughs all arranged at the same height side by side with lateral offsets, said troughs being supported by a bulky structure resting on the sea bottom and by a bulky structure suspended from floats and anchored to the bottom of the sea.
Those constraints involve spreading out the working zone and limiting the number of flexible bottom-to-surface connections that can be connected to a common floating support, via its sides, in order to avoid interference between the various flexible connections and the various floats.
That is why it is desired to provide an installation suitable for making it possible from a given floating support to use a plurality of flexible type bottom-to-surface connections, with reduced size and movement, and that is also as simple as possible to lay, being suitable for being built up at sea from a pipe laying ship.